In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and printing ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (photosensitive layer, image-recording layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by conducting plate making according to a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then removing the unnecessary portion of the image-recording layer by dissolving with an alkaline developer or an organic solvent thereby revealing the hydrophilic surface of support to form the non-image area while leaving the image-recording layer in the portion for forming the image area.
In recent years, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized, and various new image outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate (CTP) technique of carrying digitalized image information on highly converging radiation, for example, laser light and conducting scanning exposure of a lithographic printing plate precursor with the light thereby directly producing a lithographic printing plate without using a lith film. Thus, it is one of important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
Further, in the hitherto known plate making process of lithographic printing plate precursor, after exposure, the step of removing the unnecessary portion of the image-recording layer by dissolving, for example, with a developer is required. However, in view of the environment and safety, a processing with a developer closer to a neutral range and a small amount of waste liquid are problems to be solved. Particularly, since disposal of waste liquid discharged accompanying the wet treatment has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, the demand for the resolution of the above-described problems has been increased more and more.
Thus, the decrease in alkali concentration of developer and the simplification of processing step have been further strongly required from both aspects of the consideration for global environment and the adaptation for space saving and low running cost. However, since hitherto known development processing comprises three steps of developing with an aqueous high alkali solution having pH of 11 or more, washing of the alkali agent with a water washing bath and then treating with a gum solution mainly comprising a hydrophilic resin as described above, an automatic development processor per se requires a large space and problems of the environment and running cost, for example, disposal of the development waste liquid, water washing waste liquid and gum waste liquid still remain.
In the case of performing a hitherto known development processing with the high alkali developer, removal of the photosensitive layer in the non-image area is carried out mainly by dissolving a binder polymer contained in the photosensitive layer with the high alkali developer. On the other hand, in the case of performing a development processing using a low alkali developer, for example, a developer having pH of 10 or less, removal of the photosensitive layer in the non-image area is carried out mainly by dispersing a binder polymer contained in the photosensitive layer with the low alkali developer. Therefore, for example, when the development processing is performed for a long period of time, deposit (development scum) generates along with the increase of the photosensitive layer component removed in the developer. Such a development scum not only adheres to a member, for example, a developing tank of an automatic development processor but also adheres to a lithographic printing plate after the development processing, thereby causing the occurrence of stain on a printed material.
In order to solve such a problem, a method in which development and gumming of an image-exposed lithographic printing plate precursor are carried out by a single processing using a gum processing device having a plurality of gum processing units in the transporting direction is proposed in Patent Document 1. However, even in the method, the problem causing by the development scum generated in the final gum processing unit exists in the case where the processing has been continued for a long period of time.
Further, in case of a lithographic printing plate precursor having a protective layer on the photosensitive layer, it is also necessary to perform removal of the protective layer. The removal of the protective layer can be carried out separately by a water washing treatment prior to the development processing, but taking the simplification of processing steps and space saving into consideration, it is desired to also remove the protective layer in the development processing step. Therefore, it is necessary to solve the problem of development scum resulting from the protective layer component.
With respect to a polymer used in the protective layer, for example, in Patent Document 2, a lithographic printing plate precursor of on-press development type having an overcoat layer containing a specific compound having a phosphoric acid group or a phosphoric monoester group or a hydrophilic resin having a phosphoric acid group or a phosphonic acid group together with an inorganic stratiform compound is described. Also, in Patent Document 3, a lithographic printing plate precursor having a protective layer containing polyvinyl alcohol having a saponification degree of 91% by mole or more, a specific amount of a mica compound and a water soluble copolymer containing (meth)acrylic acid or its ester is described. Further, in Patent Document 4, a lithographic printing plate precursor having a protective layer containing a specific polymer of (meth)acrylic ester having a long ethylene oxide chain or propylene oxide chain in its side chain together with a stratiform compound is described.
However, the protective layers containing these polymers are not sufficient for providing a lithographic printing plate excellent in stain resistance and ink receptivity while preventing from the occurrence of development scum without causing decrease in a development speed.